[asterisk-commits] branch oej/t38passthrough r12503 - /team/oej/t38passthrough/doc/

[asterisk-commits at lists.digium.com]

Author: oej
Date: Sun Mar 12 03:13:04 2006
New Revision: 12503

URL: http://svn.digium.com/view/asterisk?rev=12503&view=rev
Log:
Update documentation

Modified:
    team/oej/t38passthrough/doc/sipt38support.txt

Modified: team/oej/t38passthrough/doc/sipt38support.txt
URL: http://svn.digium.com/view/asterisk/team/oej/t38passthrough/doc/sipt38support.txt?rev=12503&r1=12502&r2=12503&view=diff
==============================================================================
--- team/oej/t38passthrough/doc/sipt38support.txt (original)
+++ team/oej/t38passthrough/doc/sipt38support.txt Sun Mar 12 03:13:04 2006
@@ -3,13 +3,46 @@
 
 By default there is now T.38 Fax over IP support for fax passthrough
 in the SIP channel. There are a number of devices in the market that
-supports T.38 faxing.
+supports T.38 faxing. This document gives some background information
+on T.38 and the implementation in Asterisk.
+
+Faxing over VoIP without T.38 is like russian roulette. On a local LAN,
+it might work if you pick a clear channel codec like G.711. Steve Underwood, member of
+the Asterisk developer team, has writen a good article about the problems involved and
+the solutions for it on his web site, the URL is http://soft-switch.org/foip.html
+
+T.38 is an ITU standard for fax over VoIP. To simplify, the idea is to decode the fax audio
+stream at the ingress point, convert it to a data stream that is not sensitive towards jitter
+or delays and encode it into audio again if needed at the other end of the call - if you
+can't convert it to an image somewhere in the middle and print it directly, or send it by
+e-mail.
+
+T.38 is an ITU standard, approved in 1998. At that point, it allowed for transport
+over UDP with UDPTL protocols or TCP. Later, RTP support was added. The implementation
+in Asterisk support UDPTL. Using UDPTL means that RTCP error reporting is not
+supported. On the other hand, RTP support also requires support for RTP forward error
+correction, FEC, which is not currently implemented in Asterisk. RTP support would
+also mean that Secure RTP could be used. RTP support would also mean improved support
+for firewalls and NAT solutions that support RTP, but not yet UDPTL.
+
 
 More information about T.38 faxing
 ----------------------------------
-Steve have published a very good article on T.38 that we encourage
-you to read at http://soft-switch.org/foip.html
+- Steve have published a very good article on T.38 that we encourage
+  you to read at http://soft-switch.org/foip.html
+- Voip-info.org: T.38 http://www.voip-info.org/wiki/view/T.38
+- Voip-info.org: Asterisk Fax http://www.voip-info.org/wiki/view/Asterisk+fax
 
 Contributors
 -------------
-The T.38 support in Asterisk was developed by Steve Underwood
+- The T.38 support in Asterisk was developed by Steve Underwood.
+	http://soft-switch.org/
+
+References
+----------
+- ITU-T Recommendation T.38, "Procedures for real-time Group 3 facsimile
+  communications over IP networks", 2002 with Amendment 2, January 2005
+- RFC 2733, An RTP payload format for generic forward error correction
+  J. Rosenberg, H. Schulzrinne, December 199
+- RFC 3362, Realtime Facsimile (T.38) - image/T38 MIME Sub-type Registration
+